Like other secretory proteins, lysosomal proteins are synthesized in the endoplasmic reticulum and transported to the Golgi apparatus. However, unlike most other secretory proteins, the lysosomal proteins are not destined for secretion into extracellular fluids but into an intracellular organelle. Within the Golgi, lysosomal proteins undergo special processing to equip them to reach their intracellular destination. Almost all lysosomal proteins undergo a variety of posttranslational modifications, including glycosylation and phosphorylation via the 6' position of a terminal mannose group. The phosphorylated mannose residues are recognized by specific receptors on the inner surface of the Trans Golgi Network. The lysosomal proteins bind via these receptors, and are thereby separated from other secretory proteins. Subsequently, small transport vesicles containing the receptor-bound proteins are pinched off from the Trans Golgi Network and are targeted to their intracellular destination. See generally Kornfeld, Biochem. Soc. Trans. 18, 367-374 (1990).
There are over thirty lysosomal diseases, each resulting from a deficiency of a particular lysosomal protein, usually as a result of genetic mutation. See, e.g., Cotran et al., Robbins Pathologic Basis of Disease (4th ed. 1989) (incorporated by reference in its entirety for all purposes). The deficiency in the lysosomal protein usually results in harmful accumulation of a metabolite. For example, in Hurler's, Hunter's, Morquio's, and Sanfilippo's syndromes, there is an accumulation of mucopolysaccharides; in Tay-Sachs, Gaucher, Krabbe, Niemann-Pick, and Fabry syndromes, there is an accumulation of sphingolipids; and in fucosidosis and mannosidosis, there is an accumulation of fucose-containing sphingolipids and glycoprotein fragments, and of mannose-containing oligosaccharides, respectively.
Glycogen storage disease type II (GSD II; Pompe disease; acid maltase deficiency) is caused by deficiency of the lysosomal enzyme acid .alpha.-glucosidase (acid maltase). Three clinical forms are distinguished: infantile, juvenile and adult. Infantile GSD II has its onset shortly after birth and presents with progressive muscular weakness and cardiac failure. This clinical variant is fatal within the first two years of life. Symptoms in adult and juvenile patients occur later in life, and only skeletal muscles are involved. The patients eventually die due to respiratory insufficiency. Patients may exceptionally survive for more than six decades. There is a good correlation between the severity of the disease and the residual acid .alpha.-glucosidase activity, the activity being 10-20% of normal in late onset and less than 2% in early onset forms of the disease (see Hirschhorn, The Metabolic and Molecular Bases of Inherited Disease (Scriver et al., eds., 7th ed., McGraw-Hill, 1995), pp. 2443-2464).
Since the discovery of lysosomal enzyme deficiencies as the primary cause of lysosomal storage diseases (see, e.g, Hers, Biochem. J. 86, 11-16 (1963)), attempts have been made to treat patients having lysosomal storage diseases by (intravenous) administration of the missing enzyme, i.e., enzyme therapy. For lysosomal diseases other than Gaucher disease the evidence suggests that enzyme therapy is most effective when the enzyme being administered is phosphorylated at the 6' position of a mannose side chain group. For glycogenesis type II this has been tested by intravenously administering purified acid .alpha.-glucosidase in phosphorylated and unphosphorylated forms to mice and analyzing uptake in muscle tissue. The highest uptake was obtained when mannose 6-phosphate-containing enzyme was used (Van der Ploeg et al., Pediat. Res. 28, 344-347 (1990); J. Clin. Invest. 87, 513-518 (1991)). The greater accumulation of the phosphorylated form of the enzyme can be explained by uptake being mediated by a mannose-6-phosphate receptor present on the surface of muscle and other cells.
Some phosphorylated lysosomal enzymes can, in theory, be isolated from natural sources such as human urine and bovine testis. However, the production of sufficient quantities of enzyme for therapeutic administration is difficult. An alternative way to produce human acid .alpha.-glucosidase is to transfect the acid .alpha.-glucosidase gene into a stable eukaryotic cell line (e.g., CHO) as a cDNA or genomic construct operably linked to a suitable promoter.
Mammalian cellular expression systems are not entirely satisfactory for production of recombinant proteins because of the expense of propagation and maintenance of such cells. An alternative approach to production of recombinant proteins has been proposed by DeBoer et al., WO 91/08216, whereby recombinant proteins are produced in the milk of a transgenic animal. This approach avoids the expense of maintaining mammalian cell cultures and also simplifies purification of recombinant proteins.
Although the feasibility of expressing several recombinant proteins in the milk of transgenic animals has been demonstrated, it was unpredictable whether this technology could be extended to the expression of lysosomal proteins containing mannose 6-phosphate. Because typical secretory proteins like the milk proteins do not contain mannose groups phosphorylated at the 6' position, it was uncertain whether these cells possessed the necessary complement and activity of enzymes for phosphorylation of substantial amounts of an exogenous lysosomal protein. Further, if such cells did possess the necessary complement of enzymes, it would have appeared likely that phosphorylation would target the phosphorylated lysosomal protein via the mannose 6-phosphate receptor to an intracellular location rather than to an extracellular location. Substantial intracellular accumulation of a lysosomal protein might have been expected to have harmful or fatal consequences to the mammary gland function of the transgenic animal.
Notwithstanding the above uncertainties and difficulties, the invention provides inter alia healthy transgenic mammals secreting authentically phosphorylated lysosomal proteins in their milk.